Catalytic cracking processes convert hydrocarbon charges into lighter products such as gasolines. To begin with, the charges are quite light like gas oils, for example, and in order to obtain a maximum conversion efficiency from the very active zeolite catalysts it is necessary to draw off the maximum amount of coke deposited on the catalysts which rendered them less active during a regeneration stage at a temperature of between 520.degree. and 800.degree. C.
Due to the pressing for fuels, those within the refining industry have become interested in increasingly heavy charges comprising hydrocarbons with a high boiling point, such as a boiling point which is above 550.degree. C., for example, and with a high Conradson carbon or a significant metal concentration. A large amount of coke and hydrocarbons can thus become deposited on the catalyst during the catalytic cracking phase, and regeneration of the catalyst by combustion can cause significant heat discharge which can adversely affect the apparatus and render the catalyst inactive, particularly during lengthy exposure to temperature above 800.degree. C. Controlled regeneration of the catalyst is therefore imperative. The problem occurs in particular when a process involving a technique in existence for a long time which basically treats conventional hydrocarbon charges is used for much heavier charges.
One of the aims of the invention is therefore to propose a regeneration process and apparatus with controlled cooling of the catalyst in a catalytic cracking unit with a view to treating heavy charges.
Another object of the invention is to make an apparatus easier to use.
The prior art is illustrated by the following patents:
U.S. Pat. No. 4,614,726 discloses an apparatus which has a regenerator, wherein the regeneration temperature is controlled by an external heat exchanger with descending flow through a bundle of tubes.
The cooled catalyst is recycled to the regenerator through a conduit for circulating the catalyst upwardly in fluidized state, in the dense bed of this regenerator. The catalyst in the exchanger is kept in the dense bed by a fluidization gas which flows counter-currently to the direction of flow of the catalyst, and fluidization gas is either entrained with it when flow is very weak, or is removed via the intake line for the catalyst. This counter-current circulation of the gas disrupts flow of the catalyst in the intake tube and in the exchanger, and the heat exchange is not at a maximum.
U.S. Pat. No. 4,443,245 discloses a regenerator on two levels, comprising an external exchanger with lateral intake of the hot catalyst coming from the upper level which is a storage zone.
The cooled catalyst is recycled through a conduit, which receives regeneration air and the used up catalyst, in a zone at the lower level where combustion takes place. Therefore, functioning of the regenerator and of the exchanger are closely connected since the return of the cooled catalyst to the regenerator is dependent on the flow of fluidization air used for regeneration and which circulates in said conduit. This patent also discloses a small tube above the exchanger which opens into the dense bed of the exchanger in such a way that discharge of the gas and fumes cannot be complete in view of the presence of the catalyst in this tube. The catalyst circulation with backmixing phenomenon can then appear. Discharge of the gas deteriorates as the exchange bundle meets the upper end of the exchanger. The mixture does not have to be homogeneous, and therefore an upper zone exists where the catalyst stagnates and where it is not properly regenerated. This mans that heat exchange is reduced.
U.S. Pat. No. 4,923,834 discloses a "backmixing" process where an upper tube opening into the intake conduit into the catalyst exchanger which circulates in dense bed enables the catalyst to be returned from the exchanger into the storage chamber of the regenerator. This patent is therefore concerned with cooling by "backmixing and not with a solution to a problem connected with the removal from a heat exchanger of fumes and fluidization air permitting optimization of the heat exchange operation.
Finally, the prior art is illustrated by the patent French Patent 2628342 (U.S. Pat. No. 4,965,232) which discloses an external system for cooling the catalyst in a unit comprising a double regeneration of the spent catalyst providing for a catalytic cracking apparatus, the two regenerators providing for separate removal of the combustion effluents, the catalyst circulating from the second regenerator to the first via the heat exchanger. The technical problem is also concerned with finding a maximum heat exchanger. In fact, the catalyst is not supplied properly to the heat exchanger through an inclined conduit because of a quasi-absence of space for release of the fluidization gas from the catalyst in the exchanger, which means that the fluidization gas tends to rise in the conduit in the form of bubbles, therefore acting against flow of the catalyst.
The present invention aims to remedy the drawbacks mentioned hereinabove and to permit significantly improved results.